Wettability is a surface phenomenon that occurs at a boundary line between phases, one of which is a solid body, and the others are incompatible liquids or liquid and gas. Wettability appears in full or partial spreading of liquid over a solid surface or in bonding to this surface in the presence of another liquid immiscible with the former one. Wettability is an important parameter in many industries, such as pharmaceutical, light industry, oil and gas industry, etc.
For example, in the oil and gas industry, reservoir wettability is one of the key factors for determining a location, flows and distribution of liquids in a reservoir. Being the key factor for determining distribution of liquids in the reservoir, wettability affects many types of geophysical and petrophysical studies of the reservoir—electrical properties, capillary pressure curves, relative permeability, etc. Rock wettability has a significant impact on the choice of optimal oil recovery methods, especially during secondary and tertiary oil production methods.
Wettability is often classified according to the average affinity of liquids to a surface, for example, in the oil and gas industry there are: a water-wetted rock, neutral wettability or an oil-wetted rock; and according to spatial distribution of wettability, for example, in the oil and gas industry: homogeneous, fractional (heterogeneous surfaces with random distribution of rock particles having different wettability), mixed wettability (for example, fine pores are better wetted with water, and coarser pores are better wetted with oil), etc.
Wettability is determined by physicochemical interactions of fluids with each other and with the heterogeneous surface of solid media. Due to complexity of study of surface interactions, up to date there is no universal method for determining wettability. The complex spatial distribution of wettability on the surface of a porous rock is a well-known fact. Determination of the nature of wettability distribution is one of the important tasks in the oil and gas industry.
In most industries the main method used for evaluating wettability is a method of measuring a contact angle between a solid surface and a phase interface of two wetting liquids (see, e.g., W. Abdallah et. al., Fundamentals of Wettability, Oilfield review, vol. 19, no. 2, pp. 44-61, 2007). The disadvantage of this method is the long time required to define an equilibrium wetting angle (up to 1000 h.), the hysteresis of a contact angle conditioned by many reasons such as, but not limited to: a surface structure, surface irregularities, etc. Another disadvantage of the method is that the method is applicable for evaluation of wettability of smooth homogeneous surfaces and it is quite difficult to be realized for characterization of surfaces of porous media and heterogeneous surfaces.
In the oil and gas industry, wettability of a reservoir heavily wetted with water or heavily wetted with oil can be determined, in some cases, by the results of the borehole geophysical research. In most cases, the laboratory research of a rock core is used to study wettability. The USBM methods and the Amott-Harvey method or combinations thereof are considered to be industry-specific standards for evaluating wettability of a core (see, e.g., E. Donaldson and W. Alam, Wettability, Houston: Gulf Publishing Company, 2008, pp. 2-27). Both these methods are based on laboratory simulation of processes of oil displacement with water and water displacement with oil taking place in a stratum. As a result of experiments on determining wettability using these methods, only one number representing wettability: from −1 to +1 for the Amott-Harvey method and from −∞ to +∞ for the USBM method is found, as a rule, for each core sample. This number does not represent wettability directly, it rather represents the efficiency of displacement of liquids in said processes and, due to the fact that the displacement efficiency depends on wettability, this parameter is related to the state of wettability. At the same time, the resulting number nor allows determining a liquid contact wetting angle on the surface nor gives any information about distribution of wettability on the surface of the core. The complex internal pore structure has an additional effect on the test results regardless of wettability state of the surfaces.
Most of the mentioned methods make it possible to determine the average wettability, while wetting heterogeneity is not evaluable by these methods. As we can ascertain, no method of evaluating spatial heterogeneity of wettability has been nowadays proposed in the oil and gas industry.